The present invention relates generally to the design and construction of a combination bearing system for supporting a rotor shaft within a gas turbine engine. More particularly, in one embodiment of the present invention an auxiliary bearing is coupled with an active magnetic bearing to support a rotatable shaft within the gas turbine engine. Although, the present invention was developed for use in a gas turbine engine, certain applications may be outside of this field.
It is well known that a gas turbine engine must have a compressor component that develops some or all of the pressure rise specified by the system cycle. The compressor is driven by a rotating shaft connected to a turbine. Turbines are well known for converting thermal energy from a high temperature gaseous flow stream into mechanical energy. While compressors and turbines are very distinct high speed rotating machines they both utilize rows of vanes and blades to influence the fluid flow. The blade and vane rows often operate in an unsteady flow, where both the velocity magnitude and direction fluctuate. Further, individual blades may be subjected to lift and drag forces, they stall, they generate boundary layers, wakes, and under some circumstances shock waves. These high speed rotating blades are coupled to a shaft that is supported within a mechanical housing by a bearing system. The bearing system must be able to withstand significant dynamic and static loads within a hostile environment.
As engine designers continue to increase the efficiency and power output from gas turbine engines the application of magnetic bearings for supporting and controlling the rotor becomes desirable. The integration of magnetic bearings into the engine would allow the rotor shaft to be supported by magnetic forces, eliminate frictional forces, along with mechanical wear and the lubrication system. However, magnetic bearings require a back-up bearing system for supporting the rotor shaft when the magnetic forces are less than required for the support of the shaft or when the magnetic bearings malfunction.
One prior type of back-up bearing utilized with magnetic bearings has a rolling element bearing that is mounted within the engine housing concentrically with the centerline of the shaft. During normal operation the magnetic bearing supports the shaft such that there is clearance between the back-up bearing and the shaft. Upon failure of the magnetic bearing or during periods of high shock loading the shaft is moved into contact with the back-up bearing.
One limitation associated with the above back-up bearing is that the shaft must transition through an air gap in order to be supported by the back-up bearing. In a high speed rotating device, when the magnetic support is removed the shaft is radially accelerated toward the inner race of the back-up bearing, where impact, bounce, and roll all occur. Further, the transition from the magnetic bearing to the back-up bearing is destabilized by the dynamics associated with the rotating shaft engaging the stationary back-up bearing. The destabilization is due to the "deadband" effect which encompasses the impact forces generated as the unsupported shaft contacts the inner bearing race of the bearing, and the rolling friction caused by the difference in surface speed between the shaft surface and the accelerating inner bearing race of the bearing element. The non-liniarity caused by this "deadband" effect is extremely destabilizing and can result in unstable chaotic behavior.
Although the prior techniques utilizing magnetic bearing systems with back-up bearings are steps in the right direction, the need for additional improvements still remains. The present invention satisfies this need in a novel and unobvious way.